The present invention relates generally to the production of electrical steels, and more specifically to cold rolled, batch annealed and temper rolled motor lamination steels having good processing and magnetic properties, including low core loss and high permeability.
Desired electrical properties of steels used for making motor laminations are low core loss and high permeability. Those steels which are stress relief annealed after punching also should have properties which minimize distortion, warpage and delamination during the annealing of the lamination stacks.
Continuously annealed, silicon steels are conventionally used for motors, transformers, generators and similar electrical products. Continuously annealed silicon steels can be processed by techniques well known in the art to obtain low core loss and high permeability. Since these steels are substantially free of strain, they can be used in the as-punched condition (in which the steel as sold is commonly referred to as fully processed) or if better magnetic properties are desired the steel can be finally annealed by the electrical apparatus manufacturer after punching of the laminations (in which case the steel as sold is commonly referred to as semi-processed) with little danger of delamination, warpage, or distortion. A disadvantage of this practice is that the electrical steel sheet manufacturer is required to have a continuous annealing facility.
In order to avoid a continuous annealing operation, practices have been developed to produce cold rolled motor lamination steel by standard cold rolled sheet processing including batch annealing followed by temper rolling. In order to obtain the desired magnetic properties of high permeability and low core loss, it has been considered necessary to temper roll the steel with a heavy reduction in thickness on the order of 7%. Electrical steels processed by batch annealing and heavy temper rolling followed by a final stress relief anneal after the punching operations develop acceptable core loss and permeability through a complete recrystallization process. Unfortunately, the heavy temper rolling necessary for development of magnetic properties often results in delamination, warpage and distortion of the intermediate product when it is annealed, to the degree that it is unsuitable for service.
Fully-processed electrical steels are used by customers in the as-punched/stamped condition without a subsequent annealing operation being required. Standard cold-rolled electrical steels are unsuitable for most fully-processed applications due to strain remaining in the material. Fully processed materials are produced utilizing continuous anneal lines since no additional strain is required to provide acceptable flatness. Batch annealed materials, however, do not have acceptable flatness and require some strain simply to provide a flat product, which generally degrades the magnetic properties beyond a usable range. This strain is usually provided by conventional temper rolling.
For traditional cold-rolled motor lamination electrical steels, magnetic property performance is measured by standard Epstein testing at a nominal induction of 1.5 Tesla. However, once incorporated into an electrical device, the steel is not magnetically optimized for use at operating inductions below 1.5 Tesla.
Conventional hot rolling practices for non-hot band annealed cold-rolled motor lamination electrical steels have used high hot rolling finishing temperatures typically accomplished in the austenite region, and high coiling temperatures to promote "self-annealing" of the generated hot band. This practice has been previously determined to produce optimal magnetic properties. However, for some steel products specifically requiring improved cleanliness levels, this practice is unsatisfactory due to the formation of heavy subsurface oxidation. Using lower coiling temperatures has traditionally degraded magnetic properties, specifically permeability levels, due to less time-at-temperature for self-annealing.